Synthetic drugs for treating tuberculosis (TB) have been available for over half a century, but incidences of the disease continue to rise world-wide. In 2004, it is estimated that 24,500 people developed active disease and close to 5,500 died each day from TB (World Health Organization, Global Tuberculosis Control: Surveillance, Planning, Financing. WHO Report 2006, Geneva, Switzerland, ISBN 92-4 156314-1). Co-infection with HIV is driving the increase in incidence (Williams, B. G.; Dye, C. Science, 2003, 301, 1535) and the cause of death in 31% of AIDS patients in Africa can be attributed to TB (Corbett, E. L.; Watt, C. J.; Catherine, J.; Walker, N.; Maher D.; Williams, B. G.; Raviglione, M. C.; Dye, C. Arch. Intl. Med., 2003, 163, 1009, Septkowitz, A.; Raffalli, J.; Riley, T.; Kiehn, T. E.; Armstrong, D. Clin. Microbiol. Rev. 1995, 8, 180). When coupled with the emergence of multi-drug resistant strains of Mycobacterium tuberculosis (MDR-TB), the scale of the problem is amplified. It is now more than a decade since the WHO declared TB “a global health emergency” (World Health Organization, Global Tuberculosis Control: Surveillance, Planning, Financing. WHO Report 2006, Geneva, Switzerland, ISBN 92-4 156314-1).
The limitations of tuberculosis therapy and prevention are well-known. The current available vaccine, BCG was introduced in 1921 and fails to protect most people past childhood. Patients who do become infected with active disease currently endure combination therapy with isoniazid (INH), rifampin, pyrazinamide and ethambutol for two months and then continue taking isoniazid and rifampin for a further four months. Daily dosing is required and poor compliance drives the emergence and spread of multi-drug-resistant strains, which are challenging to treat. A recently-published detailed review discusses many aspects of TB such as pathogenesis, epidemiology, drug discovery and vaccine development to date (Nature Medicine, Vol 13(3), pages 263-312).
Shorter courses of more active agents which can be taken less frequently and which present a high barrier to the emergence of resistance, i.e. agents which are effective against multi-drug resistant strains of TB, are urgently required. There is therefore a need to discover and develop new chemical entities to treat TB. Recent synthetic leads are reviewed in: Ballell, L.; Field, R. A.; Duncan, K.; Young, R. J. Antimicrob. Agents Chemother. 2005, 49, 2153.
Lipid metabolism is especially important for the genus Mycobacterium and it represents a well-validated target for the development of selective antitubercular agents. The enzyme which is denoted “InhA” is an NADH-dependent (dependent on the reduced form of nicotinamide adenine dinucleotide), 2-trans enoyl-ACP (acyl carrier protein) reductase of the type 2 fatty acid synthesis (FASII) pathway in Mycobacterium tuberculosis. There is a strong body of evidence indicating that InhA is the primary target of the frontline antitubercular drug isoniazid (INH). Clinical isolates as well as laboratory modified mycobacteria over-expressing InhA show resistance to INH. The drug inhibits InhA enzymatic activity inducing an accumulation of saturated C24-C26 fatty acids and blocking the production of longer molecules, including mycolic acids. This inhibition correlates with mycobacterial cell death.
The essentiality of InhA has also been demonstrated by the use of temperature-sensitive mutants of InhA in Mycobacterium smegmatis, where a shift to the non-permissive temperature results in rapid lysis and cell death.
INH is a bactericidal drug, showing specific activity against Mycobacterium tuberculosis, and is part of the first-line drug combination regimen for antitubercular therapy. INH is activated within the mycobacterial cell by the KatG catalase. The activated form is thought to react covalently with NADH within the InhA active site to form an inhibitory adduct. X-Ray structures of InhA bound to several inhibitors are available and are being used to design new inhibitors.
In vitro-activated INH forms adducts with NAD(P) cofactors which bind to and inhibit InhA and other enzymes like DHFR (dehydrofolate reductase); the physiological relevance of these interactions in vivo is clear in the case of the enoyl-reductase, but it has been shown to be almost irrelevant in the case of DHFR; the potential role of other possible targets of INH in the antitubercular activity of the drug seems minimal or non-existent.
Resistance to INH has been associated with at least five different genes (KatG, InhA, ahpC, kasA, and ndh); 60-70% of resistant isolates can be directly linked to defects in the KatG gene (often with compensatory mutations in other genes) and less commonly in the InhA structural gene and upstream promoter region.
It is anticipated that a drug targeted at InhA, not requiring activation by KatG, would interact with the enzyme in a different way from the complex NAD-INH, would also have a different pharmacological profile from INH, would kill the majority of the current INHR (isoniazid-resistant) strains and would replace INH in the existing therapy against Mycobacterium tuberculosis. 